|  | 5.3 | IP Routing Protocols
| 5.3.4 | Static and dynamic routing
Static routing protocols are hardly protocols at all. Before routing begins, the network administrator establishes static routing table mappings. These mappings do not change unless the network administrator changes them. Protocols that use static routes are simple to design and work well in environments where network traffic is predictable and network design is simple. Because static routing systems cannot react to network changes, they are generally considered unsuitable for today's large, constantly changing networks. These networks require dynamic routing protocols. Dynamic routing protocols adjust to changing network circumstances. They do this by analyzing incoming routing update messages. If a message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, prompting routers to recalculate their routing tables accordingly. Dynamic routing protocols can be supplemented with static routes where appropriate. For example, a gateway of last resort (that is, a router to which all unroutable packets are sent) may be designated. This router acts as a central storing place for all unroutable packets, ensuring that all messages are at least handled in some way.